Carrier-frequency receiving system



N. R. BLIGH ET AL 4CARRIER-FREQUENCYRECEIVING SYSTEM I t lMarh 19, 1940.

Filled Jan. 11, 1939 FIG.I.

LIN VE NTOR KORMAN mcHARaLleH FRED BLOCH ATTORNEY y zo l Patented Mar. 19, 1940 CARRIER-'FREQUENCY REoEIvINo SYSTEM Norman Ric-hara ligh and Alfieri Bloch,

Wembley, England, assig'nors to Hazeltine Corporation, a corporation ofl Delaware Application January 11, 1939, serialno. 250,369 In Great Britain October 21, 1937 7 Claims. .(01. 25o-fao) This invention relates to modulated-carrier n signal-receiving apparatus of the type adapted to receive information conveyed by a modulated- 4carrier wave and wherein 5 picked up by' an antenna.,` passjthrough one or more carrier-frequency stages, which in `the case of superheterodyne receivers includes inter-` mediate-frequency'stages, before the informay tion, which may consist of audio or, television 'i0 signals, is detected'in a detector stage andp'assed v'on tof the output stage or stages.

In such apparatus itis known that there mayw bepicked up by the antenna, together with the desired signalsnoise signals which interfere means which increase'the ratio of signal energy .to'noise energy transmitted from the vantenna to the detector and output stages of 'modulatedcarrier signal-receivingapparatus, thereby re- 25 ducing the effect of received'noise signals.

` Figs. l, 2, and 3 of the drawing are-characteristic wave forms utilized to explain theinvention while' Figs. 4 and 5 are' schematic circuit diagrams of different embodiments of the invention.

vexplained with reference to Figs. 1, 2, and`3 of the accompanying drawing. In Fig. 1 curve (a)k represents the wave form of'several cycles of an v w amplitude-modulated carrier Wave.' Dotted curve e, (b) shows, byway of example, an irregular noise signal occurring within these cycles'oi'thecarrier wave. The energy of thecarrier wave (a) varies regularly in time, being a maximum when the carrier amplitude is a maximum, for' example, 40 at P and Q, and a minimum when the carrier amplitude is` Zero, vfor example, at R.' On the at the detector input, of a carrier-wavefsignall receiving system, of allowing signals to pass only during the quarter cycleslin which the energy 55 ofthe carrier wave is a maximum.

the lincoming signals,`

with the reproduction of the desired signals and v 0U The principle underlying the invention is 'best ter'val has a lower limit beyond which no .perceptible further improvement inV performance is y obtained. f i f Fig. 2 illustrates one manner in which the said ratio may be increased. It will be understood that the intervals during which signals are allowed to pass may occur` in every cycle, or in every group of cycles of the carrier wave; thus, g5

if ,fis the frequency of the carrierl wave, the Vfrequencyat which the active intervals occur may he 2li/n, wheren may be any finite integer. 1v The n frequency at which the intervals occur must not, y, however, he too low, yfor then the higher modula- -10 tion frequencies may be lost. Maximum signal energy is applied to the detector when thefrequency. is 2f but it will be seenlater that the frequency f is, in general, preferable.v Furthermore, each interval during which4 signals are allowed to pass may be different' from onexquarter-cycle, so long as it is substantiallylessthan one half-cycle,and not substantially less than one quarter-cycle. The effective signal-to-noise ratio increases as the interval during which signals are allowed to pass decreases, but the signal energy rsupplied to the detector simultaneously f decreases ;4 in practice it is found that such in- According-to the invention modulated-carrier signal-receiving apparatus of the type speciiied'y comprises means lfor periodically opening vand closing the path of thesignalsj through the signal-translating channel-'of the `apparatus at .a point preceding the detector and .output stages,

`the frequency ofthe opening and closing being .2f/2z, where f `is the .carrier frequency and n is any finite integer, preferably small, the phase ".35,"

of the open intervalsbeingsuch that onepeak of the carrier Wave occurs during each open interval, and'each open interval being substantially shorterthan 1/27.

In order to'pr'oduce theswitching action, opening and? closing the 'signal path in the signaltranslating channel of the receiving apparatus, theremay be, included in the signal path a modulator of the typel known as y'a chopper, This type is well known in the art and requires noy detailed description. As an example, a chopper modulator may comprise a rectangular-wave modulator in` which the modulating oscillation consists of a rectangular wave which biases4 the non-linear elements ofthe modulator so as-to render them conducting and non-'conducting' during alternate half-cycles; if the frequencyof this modulating oscillation is .2f/n asafmesaid v'and its phase-is suitably related to that ofthe incoming carrier, Ithe signal path is then opened 56 during the conducting half-cycle and closed during the non-conducting half-cycle as required. The rectangular-wave modulator may comprise a multi-grid frequency changer valve of the type usual in the art, to one control grid of which is applied the incoming signal and to the other control grid of which is applied the rectangular wave or other modulating oscillation, the output being passed to the detector and output stages. Thus a wave of the form shown in Fig. 2 may be obtained by heterodyning the carrier wave of curve (a) of Fig. 1, in such a frequency changer or chopper-modulator, with a rectangular wave of the form shown in Fig. 3. The modulating oscillation of the rectangularwave modulator may be produced locally, in which case the local oscillator` is preferably electrically synchronized with the incoming carrier Wave in known manner in order to maintain the correct phase relationship A schematic circuit diagram of a system in accordance with the invention utilizing a local oscillator synchronized with the incoming carrier wave in the intermediate-frequency channel of a superheterodyne receiver 1s shown in Fig. 4. The receiver comprises,

in cascade, an antenna-ground circuit i0, II, a radio-frequency selector and amplier l2, a frequency changer or oscillator-modulator I3, an intermediate-frequency amplifier I4, a chopper-modulator I5, a second intermediate-frequency amplifier I6, a detector and audiofrequency amplifier I'I, and a sound reproducer I8. In order to provide a control bias for causing chopper-modulator I5 to become conductive periodically, there is provided a local oscillator I9 for generating oscillations of the wave form illustrated in Fig. 3. These output oscillations of local oscillator I9 are applied to chopper-modulator I5 in order to provide the switching operations. A synchronizing signal, derived from a point in the signal-translating channel of the receiver preceding chopper-modulator I5, is applied to local oscillator I9 through conductors 2U, 2| to maintain the correct phase between the .output of the locally-generated oscillations and that of the incoming signal.

Neglecting for the moment the operation of the parts of the system involved in the present invention, the system described includes the conventional features of a superheterodyne receiver.

The operation of such receiver being well understood in the art, a detailed explanation thereof is deemed to be unnecessary. Briefly, however, a desired modulated-carrier signal intercepted by antenna I0 is selected and amplied in amplifier I2 and converted in oscillator-modulator i3 to an intermediate-frequency modulated-carrier signal. This signal is amplified by intermediatefrequency amplifier I4 and translated therefrom to intermediate-frequency amplifier I6 through chopper-modulator I5. rIhe output of intermediate-frequency amplifier I5 is detected and amplified by detector and audio-frequency amplifier I1 and reproduced by loudspeaker I8 in a conventional manner.

Local oscillator I9, generating oscillations of the wave form of Fig. 3 and having the proper phase with reference to the intermediate-frequency signal, is utilized to turn the signal-translating channel of the receiver on and off at a predetermined frequency by means of choppermodulator I5 in the manner described above. This means for turning the signal-translating channel of the receiver on and off includes means responsive to the synchronizing signal derived from conductors 2D and 2I for maintaining the phase of the open interval at the optimum value with respect to the carrier-frequency signal, that is, at such value that at least one maximum of the carrier-frequency signal occurs during each open interval.

The system of Fig. 5 is generally similar to that of Fig. 4 and similar circuit elements have been given identical reference numerals. The system of Fig. 5 differs from that of Fig. 4 primarily in the method of obtaining the rectangular-control voltage for chopper-modulator I5. In order to provide this control, there is provided a highlyselective amplifier 24 and a limiter 25, coupled to the intermediate-frequency channel of the receiver. Amplifier 24 may comprise a conventional ampliier circuit together with a highly selective circuit, the selective circuit being tuned to the intermediate frequency of the system. A

sine wave of the frequency of the intermediatefrequency signal is thus provided by the output of selector 24. This sine wave is limited at the proper values by limiter 25 to provide a signal having a substantially rectangular wave form. This wave form is applied to chopper-modulator I5 to provide switching of the signal-translating channel of the receiver in the manner described above. In the circuit of Fig. 5 exact synchronism is automatically ensured. This is one reason, as aforesaid, the frequency f is preferable.

Any of the other known arrangements for reducing the effects of noise, for example, ampliinde-limiting devices, may be included in any embodiment of the invention.

While there have been described what are at present considered to be the preferred embodimens of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

. Whatis claimed is:

l. A carrier-frequency signal-receiving system comprising, a signal-translating channel, means lll in said channel for effectively opening and closing the path of signals through said channel at a predetermined frequency, the frequency of said opening and closing being 2f/n, where 1L is any finite integer, and ,1 is the carrier frequency of said signal, and means included in said iirstmentioned means for maintaining the phase of the open intervals such that one maximum of said carrier-frequency signal occurs during each open interval, each open interval being substantially shorter than 1/22.

2. A carrier-frequency signal-receiving system comprising, a signal-translating channel, means in said channel for effectively opening and closy,

ing the path of signals through said channel at a predetermined frequency, the frequency of said opening and closing being equal to the carrier frequency of said signal, and means included in said first-mentioned means for maintaining the 'f v comprising, a signal-translating channel, means comprising a chopper-modulator in said channel for effectively opening and closing the path of signals through said channel at a predetermined frequency, the frequency of said opening and closing being 21771, where n is any finite integer and f is the carrier frequency'of said signal, and means included in said first-mentioned means for maintaining thephase of the open intervals such that one maximum of said carrierrequency signal occurs` during each open inter val, each open interval being substantially shorter than '1/27. f y

74, A carrier-frequency vsignal-receiving system .complrlising, a signal-translating channel, means -comprising a vchopper-modulator in said channel for effectively opening and closing the path'of signals through said channel at a predetermined frequency, a highly-selective circuit for ,deriving from'said channel at a point preceding said modulator a voltage for controlling said opening and closing, the frequency of said opening and closing being 21g/n where n is any nite integer and f is the carrier frequency of said signal, and means included in'said mst-mentioned means for maintaining the phase of the open intervals suchvthat one maximum of said carrier-frequency signal occurs during each open interval, each open interval being substantially shorter than l/Af.

5. A carrier-frequency signal-receiving systemA comprising, a signal-translating channel, means comprising a chopper-modulator' in said channel for effectively opening and closing the path of signals through said channel at a .predetermined frequency, a highly-selective circuit tuned to the carrier frequency ofl said `signal for derivinga signal from` said channel at a point preceding said modulator, means for limiting said derived signal, and means for utilizing said limited signal lfor controlling said opening and closing, the frequency of said opening and closing being 2f/n,

where n is any'nite' integer and f is the carrier Y tiallyshorter than l/gf.4

frequency of said signal, the 4phase of the open interval being sucli that one'maximumof said carrier-frequency signal occurs during each open interval-and each open interval being substantially shorter than MJ. y

i 6. A carrier-frequency signabreceiving system comprising, a signal-translating channel, means comprising a chopper-modulator in said channel for effectively opening and closing the path of signals through said channel at a predetermined frequency, a local oscillator for generating control oscillations for said modulator, means A"for, ,i

deriving a signal 'from' said channel` at a point preceding said modulator `fdr determining the phase of said locally-generated oscillations, the

frequency of said opening and closing being 2f/1z, .where n is any nite integer and f is the carrier frequency o f said signal, the phase of the open intervals being suchl that one :maximum of said carrier-frequency signal occursduring each open interval and each open interval-being substan- 7. A carrier-frequency signal-,receiving system of the superheterodyne type comprising, an intermediate-frequency signal-translating channel, means in said channel for eifec-tively opening and closing the path of signals through said channel at a predetermined frequency, the frequency of said opening and closing being 2f/1L, Where n- 

